The supplying of sufficient quantities of high-quality drinkable water to the population and industry is becoming a problem in many regions. The demand in many instances grows every day, but the supply of existing water may remain the same or even be reduced due to pollution. Consequently, the cost of collection, transportation and treatment of water may become greater. The correct control of consumption may be of the utmost importance in conserving water.
Water meter technology has been attentive to this necessity. In the past decades there have been notable improvements in precision and reliability of water meters.
Unfortunately, the physical principles which underlie the function and requirements of the meters are fixed, which makes it difficult to improve the state of the technical aspects of such meters. The function of a conventional water meter is based on the routing of the water being measured past the moving measurement part, generally a revolving part ("rotor"), such as a wheel or turbine. This part is moved by the passing of the water, and its movement is supposedly proportional to the velocity of the flow of the water. That is to say, a fixed quantity of water, for all flow rates, is assigned to each turn of the measuring part. But this proportion does not apply over the entire range of the meter's measuring capability, because of the impact that occurs when the fluid contacts the moving part. This impact produces alternations in the flow which are not proportional to movement of the rotor, but to the second and up to the third power of the water velocity, which produces errors in the measurement.
The whole water meter system is precise due to the fact that it is low in friction with respect to the bearing which mounts the rotor. Therefore it is essential that there be no additional friction which would decrease the rotor's sensitivity. A loose fitting inevitably provokes volumetric losses due to the fact that water will escape through the loose fittings. Depending on the circulated flow, the amount of water which fails to be measured by the meter may be considerable, such failure occurring with low flow rates of water. The moving measurement part, i.e., the rotor, falls behind with respect to the flow rate, giving an incorrect measurement.
The confluence of these factors is reflected in the lack of precision of the conventional meters, especially in the low flow rate range of measurement. Organizations responsible for approving water meters generally allow or recognize, in their regulations, a higher occurrence of measuring errors in the low flow rate range of metering. This range produces up to 2.5 times as many errors as the remainder of the flow rate range. There is also an "uncontrolled" zone (not measured) from the flow rate of zero up to the rate in which the errors mentioned above is reached.
This can be graphically represented, taking into account the following factors:
Q=O or Q2: starting flow rate--minimum flow rate that the rotor needs to turn
Qmin: minimum flow rate; the lowest flow rate that can be measured in which the precision of the meter remains within the margin of error allowed by the regulations for the measuring at the lowest flow rates (.+-.5%) in European standard.
Qt: flow rate of transition, the flow rate within which the precision of the meter has passed the margin of error, to the normal for the rest of the measuring range, or up to the maximum flow rate (Qmax), whose maximum error is .+-.2% in European and AWWA standard.
"Uncontrolled zone" is the zone between the flow rate of zero and the minimum flow rate. It is the zone in which the precision of the meter is outside the margin of error. The meter doesn't measure, or it measures in a manner that is not controlled.
There are regulations for four classes of water meters: A, B, C and D (European Standard 77/33/CEE). The requirements of these meters regarding precision in low flow rates, up to rates of 15 m.sup.3 /hour (cubic meter/per hour) are represented in FIG. 2. FIG. 2 has taken, as an example, the minimum and transition flow rates, corresponding to a meter of 3 m.sup.3 /hour. This type of water meter represents a high percentage of the installed meters.
In FIG. 2 the major requirements of precision are shown according to the advance in meter classes. The requirements of the uncontrollable zone, as well as the margin of error, diminish notably.
Centering on the class with the highest requirements, class D, there exists an uncontrollable zone that goes from zero flow rate up to a flow rate of 11 liters/hour, and the largest margin of error extends to 15 liters/hour.
This situation points to the fact that low flow rates, which can be caused by small leaks in many different situations, are not measured or are measured incorrectly. This is one of the biggest problems facing water distribution companies, the discrepancy between the water actually billed to the consumers and the total amount of water actually supplied to the customers. In addition, there is no way to detect leakage in the system or leakage on the customer's premises for flows in the uncontrolled zone.
This happens because not a lot of importance is given to the multitude of small leaks that exist in private homes, and commercial and industrial areas. However, it is evident that the sum of all of these leaks translates to a large waste of a scarce natural resource.
U.S. Pat. No. 4,798,092 to Lagergren relates to a flowmeter to measure a volume of fluid per unit of time, for example, gallons per minute. In contrast, a water meter (fluid-meter) measures the quantity of a fluid, for example, the number of gallons. Lagergren generates a pulse width modulated (PWM) pulse stream in which the width of the pulses are proportioned to the flow rate. In contrast, applicant uses signal frequency (pulses per unit of time which is proportional to the number of turns of the water meter rotor). Lagergren corrects the pulse width signal to accord with the actual flow rate using a look-up table, generally in RAM (Random Access Memory).
In U.S. Pat. No. 4,301,457 to Fukui the period of the output signals of the rotor of an electronic water meter is calculated to derive the volume of water which flowed during that period. Fukui's system requires continuous calculations that result in a system which is relatively complex, costly and has high power consumption.